SCCM Reports - v_R_System vs v_GS_System

I am new to SCCM and have some basic questions I am hoping someone can help with. I am trying to create a report to show a basic list of all computers in system. However, I have done a couple of queries and get different results with each. Doing a basic query using v_R_System, I get a list of systems as I would expect. However, when I do another query and join v_GS_OperatingSystem using ResourceID, I get another list that is about 1800 units less than the basic list. I am showing query below for review. Looks like I have about 1800 machines that do not have a last boot time listed. Does anyone have any suggestions as to how to get a reliable/believable list of machines (short of a physical inventory!)?

Also - can someone explain for is basic difference between the v_R_System and v_GS_System views? I know one shows "discovered" devices while other is list from Configuration manager. Just trying to understand which to use to create reports from. Thank you...